This invention relates to improved steel profiles and to a method of making such profiles.
In many applications it is desirable or even necessary to use steel profiles having high strength characteristics. This is particularly true where high strength at low weight is required, for example in the case of motor vehicle components, hydraulic cylinders, steel building skeleton parts or where the dimensions of the profile are required for one reason or another, to be small so that the profile is inherently subjected to high stresses, as happens for example in oil country tubular goods used in the drilling of oil wells.
It is known in the prior art to use for the hot production of e.g. seamless tubes natural hard steels with higher carbon contents up to about 0.50% and/or other strength-improving alloying additives. When strips or sheets are formed into profiles, possibly into tubular shape, the use of this type of steel produces problems during cold-processing, slitting, edging and shaping to produce the profile, due to the deformation stress of the steels to the shaping operations. During the machine-welding operations, e.g. to weld together the adjacent edges of a strip to form it into a tube, other difficulties are encountered, particularly because these steels cannot be readily welded by electric resistance welding techniques. In some, therefore, it may be said that the problems encountered with the use of these steels have heretofor been insoluble.
It is true that the adequate electric resistance weldability of such steels may be avoided by using steel which has been liquid-quenched and tempered after hot rolling (see STE 70, Merkblatt 365, "Feinkornbaustaehle fuer geschweiste Konstruktionen", Teil C, Beratungsstelle fuer Stahlverwendung, Duesseldorf, first edition, 1972) or by using fine-grain steels having higher yield strength as compared to the normally-annealed fine grain structural steels (Merkblatt 365, Teil B). However, these steels will have the requisite strength which is required at delivery status but due to their high deformation stress they will still not be suitable for cold processing themselves into profiles.
In consequence, where the industry has to produce profiles such as e.g. welded tubes, the aforementioned problems have led to a limitation of the yield strength of the profiles to approximately 500N/mm.sup.2 and of the tensile strength to approximately 650N/mm.sup.2. The tubes must be produced from strip or plates of a lower hardness steel grade having a yield strength limit of up to 500N/mm.sup.2, thereupon welded and if a higher yield strength is required, be subjected to liquid hardening and tempering. Such liquid hardening and tempering, on the other hand, is a rather expensive procedure and, in addition, has the disadvantage that the previously produced higher strength characteristic is lost again if subsequently an austenitic heat treatment, for example normalizing or hot shaping or the like, must be carried out.